The invention is related to a circuit arrangement for monitoring operating conditions of a group of direct current devices, wherein the operating conditions are represented by different potentials occurring at a sampling connector of each direct current device.
More specifically the invention is directed to such a circuit arrangement wherein the direct current devices to be monitored comprise subscriber line loops composed of a subscriber station, a subscriber line and a subscriber interface circuit of a telephone exchange system.
There is provided an evaluating unit associated with a plurality of direct current devices which unit can be selectively through-connected to a sampling point of each direct current device. The evaluating unit has to sample specific voltage conditions and corresponding operating conditions of a DC device. For this reason AC voltage components, which may occur at the sampling connector have to be separated in order to avoid a false measurement. As mentioned, a subscriber line loop can be one application for such a direct current device. In such a case, the different operating conditions correspond to open-loop or closed-loop conditions of the subscriber line loop. By monitoring these conditions, switching signals can be recognized for controlling functions and operations of the PBX. One example of such signals are dial tone signals. If each subscriber line interface circuit also employs an exchange supply fed into the circuit via supply resistors, the sampling connector may be a connecting point of one wire of the subscriber line which is connected to the supply resistor not directly coupled to the plus pole of the supply voltage.
Noise or interfering voltages composed of AC voltage components may occur at such a sampling connector and which result from inductive coupling of the subscriber line to other electrical lines, like power mains. Such AC voltage components, however, may also be useful signals, as for example the ring AC voltage. In any event, it has to be guaranteed that the evaluating unit influenced by such AC voltage components may not be disturbed such that an evaluating signal is delivered which does not correspond to the present operating condition of the sampled direct current device.
It is, therefore, an object of the present invention to improve known circuit arrangements for monitoring operating conditions in such a manner that critical sampling conditions are effectively excluded.
Another object of the invention is to improve the design of the circuit arrangement in such a manner that it can be assembled in a space-saving arrangement which also is suited for a design in an integrated circuit technology.